i) Field of the Invention
The present invention relates to a print head of a wire-dot impact printer having an armature with a print needle and a core for attracting the armature, and a production method of the print head.
ii) Description of the Related Arts
In general, in a conventional wire-dot impact printer, as shown in FIG. 1, a cantilever plate spring 1 having the functions of both an armature and an energizing spring is provided, and a print needle holder 2 and a print needle 36 are secured on the front surface of the free end of the cantilever plate spring 1. A core 5 wound with a coil 4 is arranged behind the cantilever plate spring 1 corresponding to the print needle holder 2.
In this structure, when a current supply to the coil 4 of the core 5 is switched on or off, by an attractive force between the core 5 and the cantilever plate spring 1 as the armature and a restorative force of the cantilever plate spring 1 as the energizing spring, the free end of the cantilever plate spring 1 is attracted to or repelled from the core 5. By utilizing tile reciprocal motion of the cantilever plate spring 1 due to the attraction and separation, dot printing can be carried out by the print needle 36.
In the print head of this kind, a contact surface 1a of the cantilever plate spring 1 at its free end is impacted with a contact surface 5a of the core 5 at a high speed, and thus both the contact surfaces 1a and 5a suffer considerable wear. Accordingly, in order to solve this problem, a conventional print head has been proposed, as disclosed in Japanese Patent Laid-Open No. Sho 59-98867. That is, in this case, superhard films 6 and 7 composed of non-magnetic tungsten carbide are deposited on both the contact surfaces 1a and 5a of the respective cantilever plate spring 1 and the core 5 by thermal spraying so as to perform an attraction control between the cantilever plate spring 1 and the core 5 (act as a magnetism killer) and to improve a wear resistance property of the contact surfaces.
This conventional print head will be described in detail in connection with FIG. 2.
In FIG. 2, the print head is provided with a non-magnetic frame 19 and a support 10 mounted to the frame 19, and on this support 10, one end of the cantilever plate spring 1 is supported via a spacer 21. On the front surface of the free end of the cantilever plate spring 1, an armature 22 composed of a cylindrical magnetic material is staked, and on this armature 22, the print needle holder 2 for holding the print needle 36 is attached.
On the frame 19, a first yoke 14 having the cylindrical core 5 composed of ferro silicon is also secured so that the tip surface of the core 5 may face the armature 22 with a slight gap with respect to the rear surface of the cantilever plate spring 1. The coil 4 is wound around the core 5 and is coupled with an electric circuit (not shown).
Further, on the first yoke 14, a mount plate 15 for mounting a permanent magnet 16 and a second yoke 17 are integrally mounted in stacked form. The superhard films 6 and 7 are formed on the contact surfaces 1a and 5a of the cantilever plate spring 1 and the core 5. For example, the superhard films 6 and 7 are formed as follows. First, a surface active treatment is applied to the contact surfaces 1a and 5a of the cantilever plate spring 1 and the core 5, and a superhard material such as non-magnetic tungsten carbide is deposited on the surface-treated contact surfaces by a detonation flame spraying method. Then, the superhard surfaces are ground to obtain a superhard film thickness of approximately 10 to 40 .mu.m.
However, in the conventional print head, the particle size of the sprayed superhard material such as tungsten carbide of the superhard films 6 and 7 is large, such as 5 to 10 .mu.m, and the density of the same is low. Hence, when the superhard film is formed of such a superhard material, the binding force for binding superhard particles with each other is weak and thus the superhard films 6 and 7 can be readily destructed by a repeated number of impacts between the cantilever plate spring 1 and the core 5. Hence, the degradation of the printing quality is quick. Also, the dispersion of the film quality is large and the performance of the print head is dispersed with the result of low reliability of the product.
With printing speed acceleration and printing density increase, the cantilever plate spring 1 and the core 5 are required to enable much more printing. However, this can not be achieved using the above-described conventional superhard films 6 and 7.
Further, in the above-described spraying method, the parts except the core 5 must be masked during the spraying operation. Also, after the deposition of the superhard films 6 and 7, the superhard films 6 and 7 are subjected to a high accuracy grinding process in order to obtain the desired film thickness to achieve optimum as the magnetism killer and desired abrasion resistance characteristics, and many production steps are required. At the same time, a large installation is required for the spraying method and thus the production cost for the print head becomes very high.
As described above, in the print head of the conventional wire-dot impact printer wherein the superhard films 6 and 7 are formed on the contact surfaces 1a and 5a of the cantilever plate spring 1 and the core 5 by the detonation flame spraying method, sufficient abrasion resistance of the contact surfaces 1a and 5a can not be obtained and the production cost is high.